1. Field of the Invention
This invention relates generally to electrode-type glow discharge devices used in the field of thin film deposition and, more particularly, to an improved hollow cathode magnetron sputtering system that may be used for sputtering thin films onto substrates.
2. Description of Related Art
Cathodic sputtering is a well-known technique for applying a thin film of material to a substrate. The process involves placing a "target" comprised of or coated with the material to be sputtered into a chamber containing a low pressure gas. The material to be sputtered is ejected from the target by electrically connecting the target as a cathode and creating a low pressure gas discharge between the target and a nearby anode. A negative voltage is applied to the cathode at a high current level so that gas ions bombard its surface with high energy and thereby eject (sputter) atoms that will deposit on the substrate, located nearby. The process is self-sustaining because many of the incident gas ions, rather than ejecting an atom of the material to be sputtered, create a shower of electrons that collide with neutral gas atoms and create even more gas ions.
The general concept of cathodic sputtering is set forth in great detail in U.S. Pat. No. 2,146,025, issued to Penning on Feb. 7, 1939. After Penning, the advances in sputtering were few until the early 1960s when, contrary to earlier thought, it was theoretically shown that sputtering was primarily due to momentum transfer between the incident ions and the target. This new theory soon led to a variety of sputtering devices having different geometries and using magnetic fields of various shapes to direct and guide the gas discharge. Such devices are generally known as magnetron sputtering devices.
For example, U.S. Pat. No. 3,884,793, issued to Penfold et al. on May 20, 1975, discloses a magnetron sputtering device having either a solid or hollow cylindrical target and using a magnetic field that is parallel to the long axis of the cylindrical surface to maintain the gas discharge near the surface of the target. Although the Penfold et al. device was an advancement in the field, its efficient use was limited to the sputtering of long cylindrical substrates such as wires or optical fibers.
Two other patents of interest, U.S. Pat. No. 3,616,450, issued to Clarke on Oct. 26, 1971, and U.S. Pat. No. 3,711,398, also issued to Clarke on Jan. 16, 1973, relate to magnetron sputtering devices having target and magnetic field geometries that make them best suited for the sputtering of silicon wafers and the like. The devices disclosed by Clarke use a cylindrical cathode with a plurality of permanent magnets disposed circumferentially around the cathode. The magnetic fields of the Clarke devices pierce the cylindrical cathode in such a way that a toroidal gas discharge occurs along a portion of the cathode's interior. The Clarke devices are undesirable because the size and shape of the work pieces that may be sputtered are limited by the fact that sputtered atoms can only escape from the end of the cylinder. This spot-source mode of operation limits the device to stationary or batch-type usages rather than continuous pass-by usage. Moreover, the cathode must be replaced before it is completely used up because the gas discharge occurs along a limited portion of its surface.
At least two inventors approached the cathode geometry problem with planar substrates in mind. In particular, U.S. Pat. No. 3,878,085, issued to Corbani on Apr. 15, 1975; and U.S. Pat. No. 4,166,018, issued to Chapin on Aug. 28, 1979, both disclose sputtering devices that use a substantially planar cathode along with magnetic flux lines that bisect the planar cathode. In the preferred embodiments of both inventors, the magnetic flux lines are created so as to define a "race track"-like tunnel on the surface of the cathode. The Corbani/Chapin devices are very suited to coating planar substrates. However, like the Clarke devices, the cathode/magnetic field geometry is such that the cathode wears unevenly beneath the magnetic "race track."
It was the problem of uneven cathode erosion that led to U.S. Pat. Nos. 4,356,073 and 4,422,916, both issued to McKelvey on Oct. 26, 1982 and Dec. 27, 1983, respectively. McKelvey, like Corbani and Chapin, uses a magnetic field that bisects the cathode in order to maintain the glow discharge on the surface of the cathode. However, McKelvey used a cylindrical cathode rather than a planar cathode. McKelvey's solution to uneven erosion is to mechanically rotate the cylindrical cathode around a plurality of permanent magnets arranged longitudinally within the cathode. Although the McKelvey device solves the problem of uneven cathode erosion, it leads to further problems. Specifically, it requires additional drive components that are both costly and, like all mechanical things, prone to breaking down. Minimizing mechanical down time is particularly important where the sputtering device forms part of an assembly line, such as a mill-type environment for architectural glass or magnetic media.